Phyletic gradualism is a model of

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

which theorizes that most

speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution withi ...

is slow, uniform and gradual.Eldredge, N. and S. J. Gould (1972)
"Punctuated equilibria: an alternative to phyletic gradualism"
In T.J.M. Schopf, ed., ''Models in Paleobiology''. San Francisco: Freeman Cooper. p. 84. When evolution occurs in this mode, it is usually by the steady transformation of a whole

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...

into a new one (through a process called

anagenesis Anagenesis is the gradual evolution of a species that continues to exist as an interbreeding population. This contrasts with cladogenesis, which occurs when there is branching or splitting, leading to two or more lineages and resulting in separate ...

). In this view no clear line of demarcation exists between an ancestral species and a descendant species, unless splitting occurs. The theory is contrasted with

punctuated equilibrium In evolutionary biology, punctuated equilibrium (also called punctuated equilibria) is a theory that proposes that once a species appears in the fossil record, the population will become stable, showing little evolutionary change for most of ...

History

The word ''phyletic'' derives from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

φυλετικός ''phūletikos'', which conveys the meaning of a line of descent. Phyletic gradualism contrasts with the theory of

, which proposes that most evolution occurs isolated in rare episodes of rapid evolution, when a single species splits into two distinct species, followed by a long period of stasis or non-change. These models both contrast with variable-speed evolution ("variable speedism"), which maintains that different species evolve at different rates, and that there is no reason to stress one rate of change over another.

Evolutionary biologist Evolutionary biology is the subfield of biology that studies the evolutionary processes (natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life for ...

Richard Dawkins Richard Dawkins (born 26 March 1941) is a British evolutionary biologist and author. He is an emeritus fellow of New College, Oxford and was Professor for Public Understanding of Science in the University of Oxford from 1995 to 2008. An ...

argues that constant-rate gradualism is not present in the professional literature, thereby the term serves only as a

straw-man A straw man (sometimes written as strawman) is a form of argument and an informal fallacy of having the impression of refuting an argument, whereas the real subject of the argument was not addressed or refuted, but instead replaced with a false o ...

for punctuated-equilibrium advocates. In his book '' The Blind Watchmaker'', Dawkins observes that

Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...

himself was not a constant-rate gradualist, as suggested by Niles Eldredge and

Stephen Jay Gould Stephen Jay Gould (; September 10, 1941 – May 20, 2002) was an American paleontologist, evolutionary biologist, and historian of science. He was one of the most influential and widely read authors of popular science of his generation. Goul ...

. In the first edition of ''

On the Origin of Species ''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life''),The book's full original title was ''On the Origin of Species by Me ...

'', Darwin stated that "Species of different genera and classes have not changed at the same rate, or in the same degree. In the oldest tertiary beds a few living shells may still be found in the midst of a multitude of extinct forms... The Silurian ''Lingula'' differs but little from the living species of this genus". ''Lingula'' is among the few brachiopods surviving today but also known from fossils over 500 million years old. In the fifth edition of ''The Origin of Species'', Darwin wrote that "the periods during which species have undergone modification, though long as measured in years, have probably been short in comparison with the periods during which they retain the same form". Charles Darwin, 1869. ''The Origin of Species'' London: John Murray. 5th edition
p. 551

References

External links

*{{Commonscat-inline
The distinction between phyletic gradualism and punctuated equilibrium models
- by Mark Ridley Evolutionary biology Rate of evolution

History

See also

References

External links